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Optimizing the Laser Absorption Technique for Quantification of Caesium Densities in Negative Hydrogen Ion Sources U Fantz, C Wimmer

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Optimizing the Laser Absorption Technique for Quantification of Caesium Densities in Negative Hydrogen Ion Sources U Fantz, C Wimmer Optimizing the laser absorption technique for quantification of caesium densities in negative hydrogen ion sources U Fantz, C Wimmer To cite this version: U Fantz, C Wimmer. Optimizing the laser absorption technique for quantification of caesium densities in negative hydrogen ion sources. Journal of Physics D: Applied Physics, IOP Publishing, 2011, 44 (33), pp.335202. 10.1088/0022-3727/44/33/335202. hal-00644267 HAL Id: hal-00644267 https://hal.archives-ouvertes.fr/hal-00644267 Submitted on 24 Nov 2011 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Optimizing the laser absorption technique for quantification of caesium densities in negative hydrogen ion sources U Fantz1,2 and C Wimmer2 1Max-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstr.2, 85748 Garching, Germany 2Lst. f. Experimentelle Plasmaphysik, Universität Augsburg, Universitätsstr. 1, 86135 Augsburg, Germany E-mail: [email protected] Abstract. The performance of negative hydrogen ion sources, which rely on the formation of negative hydrogen ions on a surface with low work function, depends strongly on the caesium dynamics in the source. A quantitative measurement of the amount of caesium in the source during plasma-on and plasma-off (vacuum phase) is highly desirable. The laser absorption technique is optimized for the diagnostics of neutral caesium densities close to the extraction surface on which the negative hydrogen ions are generated. The setup has been simplified as much as possible utilizing also an automatic data evaluation for online measurements at high power rf sources. The setup has been tested and calibrated in a small scale laboratory experiment. The system and the analysis of the D2 caesium line at 852.1 nm is described in detail, including effects of line saturation and density depletion. The system is sensitive in the density range of 1013 – 1017 m-3 (path length of about 15 cm), allowing also for a temporal resolution of 40 ms. First very promising results from the negative hydrogen ion source are presented, such as the increase of the caesium density due to the caesium evaporation and time traces before, during, and after the discharge indicating a strong caesium redistribution. PACS: 29.25.Ni, 52.25.Os, 52.70.Kz Submitted to: Journal of Physics D: Applied Physics 1. Introduction The formation of negative hydrogen ions in hydrogen plasmas is strongly enhanced by introducing caesium to the plasma chamber [1,2]. The underlying process is the surface process, in which hydrogen atoms or positive hydrogen ions from the plasma are converted at a surface into negative hydrogen ions [3]. Since the conversion yield depends on the work function of the converter surface the work function is actively lowered by caesium deposition on metallic surfaces [4]. The work function of pure caesium with 2.14 eV is only half as high as the work function of tungsten or molybdenum [5]. Furthermore, the formation of a dipole layer on the surface of the metal can lower the work function under high vacuum conditions (p < 10-6 Pa) and a clean caesium layer to a value as low as 1.4 - 1.7 eV at a thickness below one monolayer [6]. Due to the high chemical reactivity of caesium layers small amounts of impurities, for example the residual gas, can cause a degradation of the caesium layer and thus of the work function [7, 8]. Besides the time dependence of the work function in the vacuum phase of pulsed plasma operation, the plasma itself may modify the work function by plasma surface interaction processes and eventually decrease the thickness of the layer Laser absorption for quantification of caesium densities 2 until the layer is removed. As a consequence, the stability and reproducibility of the caesium layer at the converter surface in the vacuum and the plasma phase determines the stability and the reproducibility of the negative ion current from such an ion source. Negative hydrogen ion sources are commonly used for high energy accelerators [9], for example the front end of SNS (Spallation Neutron Source, Oak Ridge) [10] or at LANSCE (Los Alamos Neutron Science Center, Los Alamos) [11], and gain more and more relevance for the neutral beam injection systems of fusion devices [12,13]. According to the requirements small negative-ion sources (diameter of several cm) with a mono-aperture extraction system are in operation with a high duty cycle for accelerators whereas for neutral beam systems large area sources (width up to 50 cm, length up to 1 m) with a multi-aperture extraction system are applied. The requirements are even more challenging for the neutral beam system of the next generation of a fusion device, namely the international fusion experiment ITER [14], being currently under construction in Cadarache, France. The large area source (90 cm width and 1.9 m height) has to provide a current density of 200 A/m2 D⎯ (280 A/m2 H⎯) homogeneously from 1280 apertures (0.2 m extraction area) and stable for 1 hour [15]. In order to meet these requirements an ambitious R&D program has started at the Max-Planck-Institut für Plasmaphysik (IPP, Garching) several years ago [16]. Three test facilities are currently available using inductively coupled plasmas powered by an rf generator at 1 MHz frequency and up to 140 kW power. The IPP prototype rf ion source has been chosen as the reference source for ITER in 2007 [17]. In both types of applications the performance of the negative ion source depends strongly on the caesium dynamics, i.e. the caesium evaporation and desorption from surfaces, which determines the formation of the caesium layer on the converter surface. Thus, monitoring of the caesium layer would be a highly desirable tool for source optimization. However, a routine and in-situ measurement of the work function of the converter surface in ion sources is not available and indirect methods to monitor the caesium dynamics have to be used instead. One example is the optical emission spectroscopy which monitors the emission of the caesium line at 852 nm close to the convertor surface as a measure for the caesium density in the plasma phase [18]. Quantification, however, needs knowledge of plasma parameters. Although this diagnostics is being routinely used in more and more systems, a deeper understanding of the caesium dynamics is still missing. As indicated by the experiments at the test facilities [19] and during experimental investigations in laboratory plasmas [8, 20], the caesium dynamics and thus the built-up of the caesium layer in the vacuum phase, i.e. between the pulses, determines the source performance. Although there are several methods for caesium detection in the vacuum phase, for example the photoelectric current measurement (work function method), a surface ionisation detector (Langmuir detector), a quartz micro balance or a mass spectrometer, they cannot be easily used in combination with plasma operation and in the harsh environment of an high power rf ion source operating at high voltage. In addition diagnostic access to the source is also very limited. A diagnostic method which offers direct access to the ground state density of a particle in the vacuum and in the plasma phase is absorption spectroscopy utilizing a resonant transition. This well known technique is being used in many applications using either white light absorption spectroscopy or (tunable diode) laser absorption spectroscopy; however the application to caesium in discharge chambers for caesium quantification is not well established so far. The white light absorption spectroscopy of the caesium resonance line at 852 nm has been already successfully applied to laboratory experiments [20], but is not suitable for routinely operating ion sources. Here, a robust and easy-to-use setup is highly desirable with automatic data acquisition and analysis for online monitoring in vacuum and plasma operation, being density sensitive over several orders of magnitude. The paper focuses on the caesium laser absorption technique and its application to ion sources. However, for comparisons with earlier measurements the white light absorption technique is presented as well. The development of the laser absorption setup at a laboratory experiment is described together Laser absorption for quantification of caesium densities 3 with the features of the data analysis and the improvements achieved in comparison to the white light absorption technique. Finally the paper presents in detail the adoption of the customized system to the ion source and reports on first successful measurements. 2. Caesium absorption Absorption spectroscopy is an active diagnostic method which is widely used for measuring densities of stable particles (atoms and molecules) in the gas and plasma phase and also gives access to the densities of radicals, ions and metastable states of particles in the plasma. Since the intensity of the signal depends on the Einstein coefficient for absorption Bki for a transition from the lower excited state k into the higher excited state i, with k as ground state, resonance lines are well suited for this diagnostics. Therefore the caesium resonance lines in the near infrared spectral range are ideal candidates for quantification of the ground state density of caesium in the vacuum and in the plasma phase. One has to keep in mind that caesium will be partly ionized in the plasma which means a caesium quantification is restricted to the neutrals only.
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